Field of the Invention
The present invention relates to a magnetic detection device, and more particularly to a magnetic detection device for detecting a magnetic field that changes with the passage of time, for instance as in the magnetic field of a rotating body on which magnets are provided.
Description of the Related Art
For instance, Japanese Patent No. 3655897 discloses a conventional magnetic detection device for detecting a magnetic field that changes with time.
In such a conventional magnetic detection device, a disc-like magnetic moving body rotates in the circumferential direction with a rotating shaft as an axis. In a case where, for instance, a rotating shaft is attached to the crankshaft of an engine or a wheel axle, a magnetic moving body rotates integrally with the crankshaft or the wheel axle. The outer peripheral surface of the magnetic moving body is magnetized in such a manner that N-poles and S-poles are disposed alternately. A detection element is disposed facing the magnetic moving body. The detection element detects changes in the magnetism of the magnetic moving body when the latter rotates. A magnetoresistive element is used as the detection element. When the magnetic moving body rotates, the magnetic moving body passes in front of the detection element, and hence changes in the magnetic field can be detected through detection, by the detection element, of the number of passes of the magnetic moving body.
FIG. 10 is a perspective-view diagram illustrating the configuration of a conventional magnetic detection device. The magnetic detection device is provided with a magnetic moving body and magnetoresistive elements. FIG. 11A and FIG. 11B are enlarged-view diagrams of a detection element portion of the magnetic detection device of the FIG. 10. FIG. 11A is a side-view diagram and FIG. 11B is a top-view diagram.
In FIG. 10, FIG. 11A and FIG. 11B, the reference sign 101 denotes a magnetic moving body. The outer peripheral surface of the magnetic moving body is magnetized in such a manner that N-poles and S-poles are disposed alternately. The reference signs 102a and 102b denote magnetoresistive elements. The reference sign 103 is a magnet that applies a bias magnetic field to the magnetoresistive elements 102a, 102b. The reference sign 102 is a processing circuit unit. The processing circuit unit 102 has a board on the surface of which a circuit is printed. The reference sign 104 is a rotating shaft of the magnetic moving body 101. The rotating shaft 104 and the magnetic moving body 101 rotate in synchrony. The magnet 103 is magnetized in a direction parallel to the rotating shaft 104 of the magnetic moving body 101, as denoted by the solid-line arrow in FIG. 11A. The magnet 103 is disposed at a given distance from the outer peripheral surface of the magnetic moving body 101. The magnetoresistive elements 102a, 102b are disposed above the magnet 103. The magnetoresistive elements 102a, 102b are disposed side by side along the circumferential direction of the magnetic moving body 101, as illustrated in FIG. 11B. The magnetoresistive elements 102a, 102b are disposed at a given spacing Le therebetween. The dashed-line arrows in FIG. 11A denote a bias magnetic field generated by the magnet 103. A combined magnetic field of the bias magnetic field generated by the magnet 103 and the magnetic field generated by the magnetic moving body 101 is present around the magnetoresistive elements 102a, 102b. The magnetoresistive elements 102a, 102b detect only a magnetic field in a plane perpendicular to the rotating shaft 104, within the combined magnetic field.
FIG. 12 is a diagram illustrating a characteristic of the magnetoresistive elements that are used in the conventional magnetic detection device. In FIG. 12, the horizontal axis is the magnetic field (A/m) that is applied to the magnetoresistive elements 102a, 102b. The vertical axis is a resistance change rate (%) of the magnetoresistive elements 102a, 102b. As illustrated in FIG. 12, the resistance value is maximum when the magnetic field applied to the magnetoresistive elements 102a, 102b is zero. On the other hand, the resistance value decreases as the value of the applied magnetic field increases.
In the conventional magnetic detection device, as described above, a bias magnetic field is applied by the magnet 103 to the magnetoresistive elements 102a, 102b. This bias magnetic field is depicted in FIG. 12 as a bias magnetic field B0. When the magnetic moving body 101 rotates with the rotating shaft 104 as an axis, the variation of the magnetic field that is applied to the magnetoresistive elements 102a, 102b by the magnetic field of the magnetic moving body 101, i.e. the operating magnetic field range, is a range extending from B1 to B2 in FIG. 12. In this case, the operating magnetic field range of the magnetoresistive elements 102a and 102b is the same as each other, since the magnetoresistive elements 102a, 102b are disposed, at the fixed spacing Le, along the rotation direction of the magnetic moving body 101.
In the conventional magnetic detection device, the processing circuit unit 102 outputs a signal corresponding to the multipolar magnetization of the magnetic moving body 101, on the basis of the changes in the resistance values of the magnetoresistive elements 102a, 102b. The processing circuit unit 102 determines a difference between the resistance values of the magnetoresistive elements 102a, 102b, and obtains an output signal Vc by performing voltage conversion of that difference. The processing circuit unit 102 further subjects the output signal Vc to waveform shaping, to obtain thereby a final output signal Vo.
FIG. 13 is an example of a timing chart illustrating the operation of the conventional magnetic detection device. In FIG. 13, (a) denotes the resistance values of the magnetoresistive elements 102a, 102b, (b) denotes the output signal Vc, and (c) denotes the output signal Vo. Further, P is the magnetic pole pitch between the N-poles and the S-poles of the magnetic moving body 101.
The magnetic field that is applied to the magnetoresistive elements 102a, 102b varies according to the rotation of the magnetic moving body 101 with the rotating shaft 104 as an axis. The resistance values of the magnetoresistive elements 102a, 102b vary as a result, as illustrated in FIG. 13(a). In FIG. 13(a), the dashed line denotes the resistance value of the magnetoresistive element 102a and the solid line denotes the resistance value of the magnetoresistive element 102b. The magnetoresistive elements 102a and 102b are disposed, at the spacing Le therebetween, along the rotation direction of the magnetic moving body 101. Accordingly, the resistance changes of the magnetoresistive elements 102a, 102b are offset from each other by a phase proportional to the spacing Le, as illustrated in FIG. 13(a). Therefore, the output signal Vc illustrated in FIG. 13(b) is obtained by determining a difference of the resistance values of the magnetoresistive elements 102a, 102b and performing conversion to voltage of the difference. The output signal Vo corresponding to the magnetic poles of the magnetic moving body 101 can be obtained, through waveform shaping, by comparing the output signal Vc with a threshold voltage Vref, as illustrated in FIG. 13(c). Herein the output signal Vc is substantially sinusoidal in a case where the magnetic pole pitch P of the magnetic moving body 101 is substantially identical to the spacing Le between the magnetoresistive elements 102a, 102b. 